1. Field of The Invention
This invention relates to photosensitive materials which are particularly suitable for use in electrophotographic processes including charging, exposing, developing and charge-removing steps and which make use of organic photoconductors and more particularly, to improvements of such photosensitive materials. The invention also relates to a method for making the photosensitive materials of the type mentioned above.
2. Description of the Prior Art
Organic photoconductors have a number of advantages over inorganic photosensitive compounds and have now been studied extensively. Such advantages include the ease in preparation of a variety of compounds exhibiting high sensitivity at different wavelengths depending on the molecular design, little or no ecological problem, and good productivity and economy. Although the organic photoconductors have problems on durability and sensitivity, these characteristic properties have been remarkably improved at present. Some organic photoconductors have now been in use as a main photosensitive material for electrophotography.
Known organic photoconductive materials usually have a double-layer structure which includes a charge generation layer capable of absorbing light to generate carriers and a charge transport layer wherein the generated carriers are transported. Known materials used to form the charge generation layer include perylene compounds, various phthalocyanine compounds, thia pyrylium compounds, anthanthrone compounds, squalilium compounds, bisazo compounds, trisazo pigments, azulenium compounds and the like.
On the other hand, the materials used to form the charge transport layer include various types of hydrazone compounds, oxazole compounds, triphenylmethane compounds, arylamine compounds and the like.
There is now a high demand for photosensitive materials for digital recording such as by laser printers wherein organic photosensitive compounds are used in a near ultraviolet range corresponding to semiconductive laser beams with a wavelength ranking from 780 to 830 nm. Accordingly, organic photosensitive compounds having high sensitivity in the above-indicated near ultraviolet range have been extensively studied and developed. In view of the sensitivity in the above UV range, organic photosensitive compounds are more advantageous than inorganic photosensitive metals or compounds.
The organic photosensitive compounds are usually employed in combination with binder resins and applied onto substrates, such as drums, belts and the like, by relatively simple coating techniques. Examples of the binder resins used for this purpose include polyester resins, polycarbonate resins, acrylic resins, acryl-styrene resins and the like.
In general, with the double-layer structure, the charge generation layer is coated in a thickness of about one micrometer in order to attain high sensitivity. The charge transport layer is applied in a thickness of 10 to 20 micrometers. From the standpoint of the physical strength and the printing resistance, the charge generation layer should generally be formed directly on the substrate and the charge transport layer is formed as a surface layer. In this arrangement, charge transport compounds which are now in use are only those which work by movement of positive holes. Thus, the known photosensitive materials with the double-layer structure are of the negative charge type.
The negative charge systems, however, have several disadvantages: (1) during negative charging, oxygen in air is attacked and converted to ozone; (2) charging does not proceed stably; (3) the system is apt to be influenced by surface properties of a substrate such as a drum. Ozone is disadvantageous in that not only ozone is harmful to human body, but also it often reacts with organic photosensitive compounds to shorten the life of the photosensitive materials. The unstable charging often invites a lowering of image quality. In addition, the great influence of the substrate surface undesirably necessitates mirror finishing of the substrate or formation of an undercoat layer, resulting in an increase of production costs.
Moreover, the double-layer structure has the problems that the fabrication process becomes complicated with a poor yield and the stability is not good owing to possible separation between the layers. To solve the problems, there has been proposed a photosensitive material which has a single-layer structure. In either case, there has been a demand for further improvements of photosensitivity and stability in repeated use wherein a charging and residual potential removing cycle is repeated.